1. Field of the Invention
The present invention pertains to an acoustic logging tool adapted to generate shear waves in an earth formation having a torsional propagation characteristic with respect to the borehole axis.
2. Background
In acoustic well logging, an array of acoustic transducer elements for both transmission and receiving of acoustic wave signals may be arranged in a sonde or tool which is disposed in the wellbore to provide information
useful in indicating certain conditions and properties of
the formation in the vicinity of the tool. The acoustic waves generated by the tool are typically characterized as compression waves, shear waves, and pseudo-rayleigh waves.
Certain formation properties and characteristics may be determined by measuring the shear wave velocity in the formation with respect to or about the borehole axis. U.S. Pat. Nos. 3,593,255 to White, U.S. Pat. No. 4,649,525 to Angona et al, U.S. Pat. No. 4,649,526 to Winbow et al, U.S. Pat. No. 4,682,308 to Chung, U.S. Pat. No. 4,698,792 to Kurkjian et al, and U.S. Pat. No. 4,700,803 to Mallett et al and European Patent Publication No. 0224372 to Mobil Oil Corporation disclose various aspects of shear wave generating and receiving apparatus for acoustic well logging tools. However, all of these references discuss shear wave tools which generate radially outwardly directed pulses of acoustic energy in different patterns to "flex" or "pluck" the formation around the wellbore by pushing it, in effect, sideways at some point and this disturbance then propagates axially. However, by torsionally deflecting the formation around the wellbore in accordance with the method and apparatus of the present invention a fundamentally different propagation mode is developed and different formation characteristics may be measured and used in analysis of formation properties for various purposes.